Author(s): B. Mutlu Sumer
Linked Author(s):
Keywords: Scour; Offshore substation; Scour protection; Physical modelling; Stone protection
Abstract: An offshore substation (OSS) is a dedicated structure that transmits energy to the shore, the energy being delivered from wind turbines to OSS through array cables, and transmitted to the shore through an export AC or (usually) a DC cable. OSS is also feasible for floating wind farms as well with water depths of O(100 m) or shallower. When placed on the sea bed, the presence of OSS structure will change the flow pattern in its immediate neighborhood, resulting in one or more of the following processes such as the contraction of flow, the formation of near-bed and near-structure vortices, the generation of turbulence, etc. These changes usually increase the local sediment transport capacity and thus lead to scour. Scour is a threat to the stability and integrity of OSSs. Stone protection is used to protect the structure against the scour where essentially the bed area underneath and around the OSS structure is covered by a protection layer. In this keynote address, the highlights of an extensive study on scour and scour protection at an OSS in the North Sea will be presented. The presentation is organized in two parts, namely (1) scour, and (2) scour protection. Regarding the former, the work associated with this part of the presentation may be summarized as follows. In a desktop study, scour at the OSS structure has been assessed, starting with the simplest case, namely the isolated pile, followed by, with increasing complexity, assessing the influence of neighboring pile (the pile-group effect), that of other near-bed elements (J tubes, horizontal members and cables), and that of global scour at the corners of the footprint of the OSS platform, the most critical locations as far as the scour is concerned. This assessment exercise has been conducted in current only, as the latter metocean condition constitutes the most unfavorable condition. To test and verify the results, a series of laboratory experiments in a physical modelling study have been carried out in a test basin, located at the ITU Hydraulic Laboratory. We note that a special attention has been paid to the similarity conditions and scaling laws, and the aforementioned physical modelling experiments have been designed according to these conditions/laws. Regarding the second part, namely, the scour protection, the work associated with this part of the presentation may, likewise, be summarized as follows. Similar to the first part of the study, scour protection for the present OSS has been designed in a desktop study whereby a two-layer protection system (comprising a top armor layer and a bottom filter layer) has been considered, taking into consideration the scour assessment made in the first part of the study. In the design of the protection layer, various criteria (related to several hydrodynamic processes such as hydrodynamic stability of armor stones, winnowing, filter criteria, bedform destabilization, edge scour, liquefaction, among others) have been checked. Similar to the first part associated with the scour study, to test and verify the results of the desktop study, an extensive series of laboratory experiments in a physical modelling exercise have been conducted in the same test basin. The metocean conditions tested in the experiments comprise various combinations of waves and current. Similarity conditions and scaling laws related to the scour-protection processes, likewise, have been worked out, and the experiments have been designed according to the latter. The presentation will also discuss future prospects (use of CFD and also cutting-edge laboratory methods) in studies of scour and scour protection at OSS or similar structures in the offshore environment.
Year: 2023